Radiotherapy consists of the application of harmful radiation to lesions (such as tumours) within the body. The radiation interferes with cellular processes within the lesion and alleviates the condition. Typically, the radiation takes the form of high-energy beams of x-radiation, with photon energies usually in excess of 1 MeV.
A number of systems are provided for monitoring the radiation that is delivered, including a so-called “portal” image, which is an image created by the treatment beam, captured using a flat-panel detector after the beam has passed through and been attenuated by the patient.
However, portal images are usually of a relatively low quality, with poor contrast within soft tissue areas. This is inherent to portal imaging, since the attenuation coefficients of the materials involved are very similar at the x-ray energies used for the treatment beam. Diagnostic imagers usually use kV beams (i.e. in the 10-200 keV range) at which the attenuation coefficients of the various materials involved are more different and thus a greater degree of contrast is possible.
US2010/0119032A1 discloses a system in which kV and MV images obtained during treatment are used to reconstruct an image of the patient's anatomy. This is then compared to a predicted dose distribution based on the machine settings and parameters during the dose delivery.